1. Field of the Invention
The present invention relates to an all-electric glass-melting deep furnace for producing glass and to a method of refining and supplying glass.
2. Description of Related Art
A conventional glass furnace consists of a bottom of fireproof bricks and a side wall constructed by piling up fireproof bricks on the perimeter of the bottom. A known furnace of this type is a side-port type tank furnace which is an open-hearth furnace for heating glass material charged into the furnace to a predetermined temperature by fuel oil burners, gas burners, or heat sources such as electrodes built in the furnace.
A conventional glass furnace is shallow or is designed with no thought given to the depth. Therefore, as production volume (flow volume) of molten glass increases, more seeds (small bubbles) are contained in the molten glass, which causes a problem.
For example, when a transparent product such as a plate glass is continuously produced, molten glass flows out through a hole formed in the bottom and is fed to a making machine through a refiner. At the same time, the same volume of glass raw materials as that of the molten glass flowing out through the hole is continuously charged into the furnace from the top thereof so that the volume of the molten glass in the furnace can be kept constant. The glass raw materials charged into the furnace are not melted at once but it is piled up on the surface of the molten glass, thus making a batch of glass (layer). In the open-hearth furnace, the batch of glass is subjected to a strong heat radiation caused by the burning of fuel oil or gas from the above. The molten glass is not fully covered with a batch of glass, but is widely exposed. Such exposed portion of the molten glass is designated as “mirror”.
In the furnace filled with a predetermined amount of molten glass, a burner is provided above the glass raw materials and a plurality of pairs of heating electrodes are supplementally provided as heat sources to the side wall or bottom of the furnace in such a manner that the heating electrodes are placed at lower positions than the surface level of the molten glass. An energy for melting the glass raw materials is supplied by main heating with fuel oil burner or gas burner placed above the glass raw materials and by supplemental heating with the heating electrodes. Therefore, when the fuel oil burner or gas burner is ignited and an electric current is passed through the electrodes, the glass raw materials are heated from above and underneath by the electrodes. An example of convection generated by such heating is as follows. Specifically, the molten glass near the heating electrodes is heated first and moves upward until it reaches right below the batch. Then, it flows horizontally to some extent and moves downward towards the bottom as a downward flow. When the batch touches such convection layer between the heating electrodes and the batch, it is heated and gradually melted.
The aforementioned molten glass is attracted by the molten glass flowing out from the throat (hole) formed in the bottom of the furnace and moves downward towards the bottom that is placed at a lower position than the heating electrodes while lowering the temperature. In the furnace, the molten glass near the batch is particularly referred to as a seed-containing layer. The molten glass immediately after being melted contains seeds composed mainly of carbon dioxide. However, these seeds are removed when the molten glass flows towards the throat or when it is in a refiner. The term “remove” used herein means that the seeds go upward in the molten glass and is released from the surface of the molten glass and that the seeds are removed (namely, the molten glass is refined) by being absorbed into the molten glass in the course of lowering the temperature.
Such techniques are disclosed in the Japanese Unexamined Patent Publication No. (Patent Kokai No.) 54-22424 (1979) and Japanese Unexamined Patent Publication No. (Patent Kokai No.) 58-32030 (1983).
In a conventional furnace, improvement of heat efficiency is hindered by the following factors. For this reason, when an emission problem of carbon dioxide and exhaustion of fossil fuel become more and more serious in the future, significantly high consumption of energy such as electric power or fuel to be provided to the heat sources is seen as a problem.
The body of the furnace is generally s like a box. If the inside dimension of the furnace is set to be large, the heat escaping upward in the furnace is significantly increased. Therefore, unnecessary heat must be provided into the furnace through the aforementioned heat source.
Further, if the inside dimension of the furnace is set to be large, a batch of glass becomes thin so that this causes too much cooling of the surface of the molten glass. As this result, it is shown by experience that the molten glass containing a lot of seeds highly tends to flow downward and the seeds go into the refiner. This is considered to be because since the flow rate increases when the surface of the molten glass is quickly cooled, there is not sufficient time to completely remove the seeds and therefore a tiny amount of seed-containing molten glass flows to the bottom of the furnace.
Further, since a batch of glass itself acts as a heat insulating material for preventing heat radiation from the surface of the molten glass, a thick batch of glass can, to some extent, prevent the presence of seeds in the refiner. However, it is technically difficult to keep the thickness of the batch constant because the batch is gradually melted as time goes by. Particularly, where the inside dimension of the furnace is large, it is necessary to melt the batch completely in the furnace and to produce a wide range of mirror portion containing fewer seeds.
Accordingly, as disclosed in the aforementioned patent publication No. 54-22424 (1979), it is tried to make the diameter of an opening of the top portion shorter than the inside dimension of the bottom of the furnace by forming a step or a narrow portion on or in the body of the furnace using an all-electric melting technique. This can reduce the amount of heat escaping upward in the furnace. However, since such step or narrow portion increases the surface area of the furnace, a desired heat efficiency cannot always be attained. Further, such problem arises that when the bricks corresponding to the step or narrow portion are exposed to high temperature, they are broken into stone pieces (stones) and mixed into the molten glass.
Alternatively, if the whole furnace is downsized, the surface area thereof can be reduced. However, since the volume of refining area is reduced, the amount of high-quality molten glass obtained at a time is decreased.
Accordingly, objects of the present invention are to provide an all-electric glass-melting deep furnace in which high-quality molten glass can be efficiently produced at high heat efficiency and to provide a method of refining and supplying glass.